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What is the Hubble Telescope?

I am embarrassed to admit that although I have heard a lot about the Hubble Telescope, I’m still not actually that sure about its purpose. Does it belong to NASA or just the world in general? Is it just a telescope that sends back picures or is it looking for extra-terrestrial life? How big is it? Is it a satellite or does it just travel for eternity into space? Is it solar powered or will it eventually run out of juice? Please help me sound like less of an idiot about this!

Cathrine

Hi Cathrine

Your question is understandable. We’ve been sending probes and robotic spacecraft out into the Solar System for so many decades that it can be hard to tell them apart. And while the Hubble gets a lot of publicity for the stunning images it returns, not a lot of technical information gets relayed through the evening news. So here are the (rather long-winded) facts:

The Hubble Space Telescope, usually referenced by the snappy acronym HST, is a modestly sized observatory placed in orbit around the Earth. The fundamental measurement of any telescope is the diameter of its primary mirror, and the HST’s mirror is 2.5 meters across. HST has revolutionized both the science of astronomy, and the business of selling astronomy posters. By being positioned up above the Earth’s atmosphere, it has a clarity of vision that has yet to be matched by earth-based observatories: Hubble can see further and in more detail than any other optical telescope built (although technologies like Adaptive Optics, which did not exist when the HST was built, are rapidly allowing modern super-telescopes to catch up).

Hubble Space Telescope orbiting Earth

Although the idea of a space telescope had been around since the 1920’s, it took over 70 years of proposals, planning and construction before HST was ready for launch in 1986. Unfortunately the launch was delayed by the tragic explosion of SS Challenger in 1986, which was intended to deliver the Hubble to orbit. It was only after it had finally been installed and the first images transmitted back to Earth that a critical manufacturing flaw was discovered: The primary mirror suffered an optical defect known as spherical aberration. That massive piece of glass had been shaped with unearthly precision, to the wrong shape. Considering that the total bill at time of launch came to $1.5 billion, somebody must have had an awful lot of explaining to do! Most telescope primary mirrors (including the HST’s) are ground down to a perfect section of a sphere, accurate to a hundredth of a micron. They are then ‘figured’, a delicate process which polishes away vanishingly small amounts of glass to change the shape to a perfect parabola. When insufficient figuring is applied, the curvature is not fully parabolized, with the result that the telescope can never come to full focus. This was the problem that was discovered, and the result was horrible blurred images. HST was not useless, of course, as it’s position above the atmosphere still gave it an advantage over ground telescopes, but it was still a devastating disappointment that such a massive project could have such a fundamental flaw. Just how far out was the shape? The centre of that 250cm mirror was thicker than it should have been by about one fiftieth the thickness of a sheet of paper. It was not until the first service mission in December 1993 that a corrective optics package could be installed (In layman’s terms: They fitted the Hubble telescope with a contact lens), allowing the HST to finally operate at its full potential.

Since then, a number of service missions have gone up, each improving and updating HST by installing new and more capable instruments and repairing aging parts. There is one final service mission scheduled, for some time in 2013. Hubble will likely eventually be decommissioned some time after that, in a controlled reentry which will see one of the most incredible scientific instruments ever built burning up in Earth’s atmosphere over the ocean.

These two images of spiral galaxy M100, taken by the HST before and after the corrective optics package was installed, show how dramatic the original flaw was

Hubble has made enormous contributions to astronomy in the 21 years since its launch. It’s position above the atmosphere gave it three distinct advantages over any telescope on Earth: It is not subject to the distortions of our turbulent atmosphere, which causes the stars to twinkle and photographs to blur. It can see in colours outside of the visual spectrum, from ultraviolet (UV) down to infra-red (IR). UV light is filtered by the Earth’s ozone layer, while IR is absorbed by water vapour, making it impractical to try and observe in these wavelengths from the ground. And finally, Hubble is in space – it can continue to observe even during daylight.

As a result of these benefits, Hubble has been able to see galaxies at such immense distances that it has vastly expanded astronomical knowledge: thanks to the HST, we now know the age of the Universe to be around 13.7 billion years (an improvement from our previous best-guess of somewhere between 10 and 20 billion years). We have found physical evidence of Dark Matter, we have discovered Dark Energy (the name given to whatever mysterious force is blowing the universe apart, so that its expansion actually speeds up over time). We have even discovered hundreds of new planets in other solar systems.

While there are now many other space observatories, some orbiting earth, others orbiting the Sun, the HST remains something special. The newer telescopes are doing fantastic work, things way beyond HST’s capabilities, but they are extremely narrowly focused on their missions. Only the Hubble can do so many different things so well. And of course, the Hubble was the first. Here’s hoping it continues to turn the world of Astronomy on its head with more and greater discoveries until it is eventually decommissioned.

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Allen is an amateur astronomer, an IT professional, a podcaster, a father of five beautiful kids and a barely competent chess player.
He is also the director of the Astrophotography Section of the Astronomical Society of South Africa, where he coordinates and promotes the activities of people who are far better photographers than him.